Is your favourite fish safe?

Salmon, rich in omega-3 oils, but consumption should be limited to one portion a week because of contamination

Oily fish may be good if you want to protect your heart, but eat too much of it and you could be filling your body with poisons and toxins, according to a new Government report. So how cautious should you be over what fish you buy? Our writer, a nutritionist, explores the options...

Mackerel

The richest supplier of omega-3 oils - linked with healthy brain development, lower risk of death from recurrent heart attack and an improvement in inflammatory conditions such as arthritis. Also very rich in vitamin B12, needed for red blood cell production and healthy nerves, and bone-building vitamin D.

One small mackerel also provides the full daily requirement of iodine - needed for manufacturing thyroid hormones that regulate metabolic rate.

It is fished from the wild and so is generally a safer bet than salmon, especially the farmed kind. However, this is not the case for huge king mackerel (prized by anglers), which are high in mercury and should be avoided by pregnant women.

The process by which mackerel is smoked adds a high level of salt which can contribute to raised blood pressure, so fresh is healthier. 10/10

Swordfish

Large predatory fish including swordfish, but also shark and marlin, can contain relatively high levels of mercury in the form of methylmercury, which may harm the nervous system of an unborn child. Infants and children are at greater risk from mercury poisoning because they eat more food relative to their body size than adults.

The official advice is that non-pregnant adults can eat the fish occasionally as part of a balanced diet - but no more than once a week. Mercury content aside, it is a low-fat, high-protein food, rich in vitamin B12 and iodine. 3/10

Cod

Low in fat and calories, and supplies very good quantities of iodine and vitamin B12. But being a white fish, it has low quantities of beneficial omega-3 oils and only a trace of vitamin D.

Cod eat other fish, so their flesh tends to show a greater concentration of pollutants. Farmed cod may be even more risky depending on the type of fishmeal they are fed. Atlantic cod is now endangered as stocks have been overfished

To be safe, look for New Zealand or Icelandic cod which is still fished sustainably and may be less polluted because it swims in purer waters.

Battering and frying bumps up its fattening potential considerably - to about 450 calories and 23g fat in a large portion. 5/10

Haddock

Extremely rich in iodine, which helps to strengthen the thyroid, with more than double the amount found in cod. It is also higher in calcium and iron than cod, although still not a very rich source of either of these minerals. One fillet supplies the

Recommended Daily Allowance (RDA) of vitamin B12.6/10

Whitebait

These tiny oily fish are eaten whole with the bones which means they supply all of the RDA of calcium in a 100g serving, and a third more iron than the same weight of rump steak. But they are usually served fried - which means one portion has up to 525 calories and

Rich in omega-3 oils, and an excellent source of vitamin B12 which helps to protect your heart. But the official advice is to limit consumption of salmon to one portion a week because of contamination with dioxins and PCBs, cancer-causing chemicals. There's no proof that organic or wild salmon are any less contaminated than farmed, but according to Friends of the Earth, this is probably the case.

To be safe, choose paler fish - it's a sign the salmon hasn't been given so much artificial colour in its feed.

A good bet is to go for canned Pacific salmon, as it is caught wild from waters that are relatively purer than those of the Atlantic. It also has a high level of vitamin D not found in Atlantic salmon because canning partially dissolves the bones into the flesh.

Smoked salmon retains all the beneficial omega-3 fats, but an average portion supplies around half of the daily recommendation for salt intake.

A really good source of omega-3 oils and - eaten with the bones - a very rich source of minerals. A serving of four sardines provides more than 50 per cent of the RDA for calcium, a third for iron, and 20 per cent for zinc (essential for healthy skin, immune and reproductive systems). However, the salt content of canned sardines is high. 5/10

Tuna

Canned oily fish is generally as healthy as fresh, but tuna is the exception. Unlike other fish it is cooked prior to the canning process, which allows the beneficial omega-3 oils to leach away. But a serving of either the canned or fresh supplies the RDA of vitamin D, and the daily recommendation of selenium - credited with reducing rates of heart disease and cancer. Slimmers should choose tuna canned in water or brine - it has about half the calorie content of tuna canned in oil. 8/10 (fresh) 6/10 (canned)

Kippers

High in salt, but richer in vitamin D and vitamin B12 than virtually any other food. Our bodies can store supplies of these vitamins for later use, which means just one large kipper fillet every one to two weeks is enough to single-handedly supply all we require. 9/10

Monkfish

Like other white fish, monkfish is low in calories and fat, but is also low in beneficial omega-3 fatty acids. It has good levels of vitamin B12, and contains metabolism-regulating iodine. 5/10

Shellfish

A reasonable source of iodine and selenium. Prawns and mussels also supply more calcium, gram for gram, than milk, and cockles are an extremely rich source of iron. A dozen oysters supply more than five times the RDA of zinc, which probably accounts for their purported aphrodisiac qualities (zinc is important for both strong sperm and female reproductive health). But people at high risk of heart disease should go easy on shellfish as it is high in sodium (from salty seawater), and cholesterol. One small lobster contains 750mg of cholesterol - whereas the maximum healthy intake is just 300mg per day. 7/10